when i write a program in assembly and i define a variable of "word" size.....is it truly the natural word size of my processor? for example on an i7 64bit processor will it be the size of 64bits.....or will it be defined as how microsoft defines the size of a words "A WORD is a 16-bit unsigned integer" ?

Its whatever Microsoft says it is. Here is a good explanation of that. The size of a word or dword is assembler dependent, not os dependent.